1. Field of the Invention
The invention generally relates to a semiconductor device and method of manufacture and, more particularly, to a semiconductor device and method of manufacture which imposes tensile and compressive stresses in the device during device fabrication.
2. Background Description
Mechanical stresses within a semiconductor device substrate can modulate device performance. That is, stresses within a semiconductor device are known to enhance semiconductor device characteristics. Thus, to improve the characteristics of a semiconductor device, tensile and/or compressive stresses are created in the channel of the n-type devices (e.g., nFETs) and/or p-type devices (e.g., pFETs). However, the same stress component, either tensile stress or compressive stress, discriminatively affects the characteristics of an n-type device and a p-type device.
In order to maximize the performance of both nFETs and pFETs within integrated circuit (IC) chips, the stress components should be engineered and applied differently for nFETs and pFETs. That is, because the type of stress which is beneficial for the performance of an nFET is generally disadvantageous for the performance of the pFET. More particularly, when a device is in tension (e.g., in the direction of current flow in a planar device), the performance characteristics of the nFET are enhanced while the performance characteristics of the pFET are diminished. To selectively create tensile stress in an nFET and compressive stress in a pFET, distinctive processes and different combinations of materials are used.
For example, a trench isolation structure has been proposed for forming the appropriate stresses in the nFETs and pFETs, respectively. When this method is used, the isolation region for the nFET device contains a first isolation material which applies a first type of mechanical stress on the nFET device in a longitudinal direction (e.g., parallel to the direction of current flow) and in a transverse direction (e.g., perpendicular to the direction of current flow). Further, a first isolation region and a second isolation region are provided for the pFET and each of the isolation regions of the pFET device applies a unique mechanical stress on the pFET device in the transverse and longitudinal directions.
Alternatively, liners on gate sidewalls have been proposed to selectively induce the appropriate stresses in the channels of the FET devices (see, Ootsuka et al., IEDM 2000, p. 575, for example). By providing liners, the appropriate stress is applied closer to the device than the stress applied as a result of the trench isolation fill technique.
Also, there have been many proposals to improve both nFET and pFET device performance using tensile and compressive stresses, respectively, which include modulating spacer intrinsic stresses and STI (shallow trench isolation) material changes individually for two MOSFETs using masks. Tensilely strained Si on relaxed SiGe has also been proposed as a means to apply this stress. Unfortunately, the tensilely strained Si on relaxed SiGe can apply only biaxial tensile stress on the Si cap as used in stack form. This constrains the regime of Ge % that is useful because of the nature of pFET sensitivity to stress. The nFET performance monotonically improves with biaxial tension; however, the pFET is degraded with biaxial tension until about 3 GPa at which point it begins to improve.
In order to improve both the pFET and nFET simultaneously, the Ge % needs to be high, approximately greater than 25-30% (or equivalent to approximately greater than 3-4 GPa in stress). This level of Ge % is difficult to implement into processes and is not very manufacturable with major issues including surface roughness, process complexity, defect and yield control, to name but a few. Given that a high Ge % is hard to use for the pFET (since it would be detrimental because of the relatively low levels of tension), other methods must be devised to improve the device performance.
Additionally, Si:C is know to grow epitaxially on Si where it is inherently tensile. A 1% C content in the Si:C/Si material stack can cause tensile stress levels in the Si:C on the order of 500 MPa. In comparison, in the SiGe/Si system about 6% is needed to cause a 500 MPa compression. This 1% level of C can be incorporated into Si during epitaxial growth as shown in Ernst et al., VLSI Symp., 2002, p. 92. In Ernst, the Si/Si:C/Si is in a layered channel for nFETs. However, the Si:C part of the structure is not relaxed. Instead, in Ernst, an unrelaxed Si:C is used as part of the channel, itself, with a very thin Si cap. The problem with this approach is that the mobility is not enhanced, but retarded, depending on the C content, from scattering.
While these methods do provide structures that have tensile stresses being applied to the nFET device and compressive stresses being applied along the longitudinal direction of the pFET device, they may require additional materials and/or more complex processing, and thus, resulting in higher cost. Further, the level of stress that can be applied in these situations is typically moderate (i.e., on the order of 100s of MPa). Thus, it is desired to provide more cost-effective and simplified methods for creating large tensile and compressive stresses in the channels nFETs and pFETs, respectively.